The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to the elimination of flow artifacts in MRI images, and particularly, the elimination of such artifacts in images acquired with echo-planar ("EPI") pulse sequences.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M.sub.z, may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B.sub.1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (G.sub.x G.sub.y and G.sub.z) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
The concept of acquiring NMR image data in a short time period has been known since 1977 when the echo--planar pulse sequence was proposed by Peter Mansfield (J. Phys. C.10: L55-L58, 1977). In contrast to standard pulse sequences, the echo-planar pulse sequence produces a set of NMR signals for each RF excitation pulse. These NMR signals can be separately phase encoded so that an entire scan of 64 views, for example, can be acquired in a single pulse sequence of 20 to 100 milliseconds in duration. The advantages of echo-planar imaging ("EPI") are well-known, and there has been a long felt need for apparatus and methods which will enable EPI to be practiced in a clinical setting. Other echo-planar pulse sequences are disclosed in U.S. Pat. Nos. 4,678,996; 4,733,188; 4,716,369; 4,355,282; 4,588,948 and 4,752,735.
In recent years, echo-planar imaging (EPI) has found increased use in both vascular and non-vascular imaging. Although the short acquisition time of "single-shot" EPI essentially freezes motion, it is susceptible to off-resonance effects and resolution limitations. To reduce these effects, "multi-shot" versions of EPI have been developed. A common multi-shot implementation is interleaved EPI, where the phase-encoding views of one shot, which provide coarse k-space sampling, lie next to the views of the next shot. Although the off-resonance effects are typically less severe than in the single-shot case, other EPI-specific artifacts, such as those due to flow and motion, can be more severe and are complicated by the interleaved nature of the multi-shot acquisition. Potential velocity-induced artifacts, which include ghosting, misregistration, and signal loss resulting from the unique gradient waveforms and long period of data acquisition, make motion compensation particularly desirable.
The theoretical effects of moving or flowing spins on echo-planar acquisitions are well known. As opposed to conventional spin-warp imaging, in which in-plane constant-velocity flow causes correctable misregistration effects, EPI is prone to severe velocity-induced artifacts. As a result, artifact reduction techniques using first order gradient moment nulling have been proposed for both single-shot and multi-shot EPI (P. M. Pattany, J. J. Phillips, L. C. Chiu, J. D. Lipcamon, J. L. Duerk, J. M. McNally, and S. N. Mohapatra, Motion Artifact Suppression Technique (MAST) for MR Imaging. J. Comput Assist Tomogr. 11(3), 369-377 (1987); J. L. Duerk and O. P. Simonetti, Theoretical Aspects of Motion Sensitivity and Compensation in Echo-Planar Imaging. JMRI. 1(6), 643-650 (1991); D. N. Firmin, R. H. Klipstein, G. L. Hounsfield, M. P. Paley, and M. P. Longmore, Echo-Planar High-Resolution Flow Velocity Mapping. Magn. Reson. Med. 12, 316-327 (1989); R. M. Weisskoff, A. P. Crawley, and V. Wedeen. Flow Sensitivity and Flow Compensation in Instant Imaging. in SMRM. 1990. New York, NM: p. 398; D. G. Nishimura, P. Irarrazabal, and C. H. Meyer, A Velocity k-Space Analysis of Flow Effects in Echo-Planar and Spiral Imaging. Magn Reson Med. 33(4), 549-556 (1995); and K. Butts, S. J. Riederer, R. L. Ehman, J. P. Felmlee, and R. C. Grimm, Echo-Planar Imaging of the Liver with a Standard MR Imaging System. Radiology. 189(1), 259-264 (1993).
Gradient moment nulling in the slice-select direction (i.e., z axis in an axial scan) in EPI is independent of k-space trajectory and identical to that used in conventional 2DFT imagining. For flow along the frequency-encoding (x) direction, where velocity-induced phase oscillates between the even and odd echoes in a given shot, gradient moment nulling can not be achieved for all echoes. However, if a unipolar, or "flyback" readout is employed and data are acquired only during one polarity of the read out gradient, all echoes can be compensated as described in D. A. Feinberg, R. T. Turner, P. D. Jakab, and M. von Kienlin, Echo-Planar Imaging with Asymmetric Gradient Modulation and Inner-Volume Excitation. Magn Reson Med. 13, 162-169 (1990).
For flow components along the phase-encoding (y) direction, where velocity-induced phase accrues quadratically over the readout period, several gradient moment nulling techniques have been proposed. For single-shot EPI, methods have been proposed for flow compensating one particular echo, or flow compensating every echo, but at the cost of scanning efficiency for the latter. The only experimental method thus far reported for multi-shot EPI uses gradient moment nulling to zero the phase of the lowest k-space view of each shot. However, as will be explained below, this single-echo gradient moment nulling method does not fully address the complications introduced by the interleaved acquisition and as a result, significant flow-induced artifacts can persist.
In single-shot EPI, because all views are acquired sequentially after a single excitation, phase due to all sources accrues monotonically and continuously over the echo train and thus smoothly across k.sub.y sampling of k-space. This applies to linear (zeroth-order) phase due to inhomogeneity and susceptibility effects as well as to quadratic (first-order) phase due to constant-velocity flow or motion. In contrast, the non-sequential (interleaved) acquisition of views in multi-shot EPI disturbs this smooth phase evolution across k.sub.y sampling. Specifically, a multi-shot EPI scan produces a zeroth-order phase map that resembles a staircase, with the number of steps equal to the number of echoes per shot. Here we define the number of echoes per shot as the echo train length ("ETL"). The "sliding echoes" technique described by F. Farzaneh and S. J. Riederer, Hybrid Imaging with Gradient-Recalled Sliding Echoes. in 7th SMRI. 1989. p.70; and D. A. Feinberg and K. Oshio, Gradient-Echo Shifting in Fast MRI Techniques (GRASE Imaging) for Correction and Field Inhomogeneity Errors and Chemical Shift. J. Magn. Reson. 97, 177-183 (1992); linearizes this off-resonance phase by adding a cumulative time delay to the echo train waveform on successive shots. For imaging static objects, this is sufficient to remove the major ghosting artifacts due to the multi-shot acquisition. However, as will be described below, when imaging in the presence of flow or motion, discontinuities in the quadratic phase across k.sub.y sampling persist and cause artifacts.